Consumer and industrial applications continue to drive demand for new and efficient batteries for use as energy sources. Important goals include obtaining more power from increasingly smaller battery packages in an environmentally respectful fashion. Envisioned applications for batteries include everything from mobile electronics to electric vehicles. Portability, rechargeability over a large number of cycles, low cost, high power, lightweight and consistent performance over widely varying loads are among the key attributes required for batteries. The specific combination of battery performance requirements varies widely with the intended application and the battery components and materials are typically optimized accordingly.
An important developing application area for rechargeable batteries is electric vehicles (EV) and hybrid electric vehicles (HEV). In these applications, the battery must have the ability to provide high currents in short time periods in order to achieve effective acceleration. High-rate discharge capability is therefore necessary. High battery power over extended time periods is also needed so that vehicles of reasonable size and weight can be maintained in motion for reasonable time intervals without recharging. Rapid recharging over many cycles should also be possible using readily available electrical power sources. The preferred cycle life profile also requires a high number of charge/discharge cycles at a low, rather than high, depth of discharge for the HEV application. Progress has been made in the development of batteries for HEV applications and two HEV automobiles have recently been made available to the U.S. public. Nonetheless, the batteries used in these automobiles represent compromises and trade-offs in relevant performance parameters and new developments are needed to further extend the capabilities of HEV and EV products.
One aspect of rechargeable batteries for HEV, EV, 42 V SLI and other applications that has received relatively little attention is low temperature characteristics. For HEV and EV products it is desirable to have batteries that perform well in winter climates. Similarly, achievement of portable and stationary power sources based on rechargeable batteries that are capable of functioning outdoors in cold climates or in indoor cold environments is also desirable. A basic limitation of virtually every battery technology is a diminution of power and performance at low temperature. The deleterious effects of temperature are especially pronounced below freezing.
Nickel metal hydride batteries have emerged as the leading class of rechargeable batteries and are replacing earlier generation nickel-cadmium batteries in many applications. Current HEV and EV products, for example, utilize nickel metal hydride batteries and expanded performance of HEV and EV products in the future are expected to depend largely on the capabilities of nickel metal hydride batteries. Like other rechargeable batteries, nickel metal hydride batteries suffer significant degradation in power and performance upon a lowering of temperature. Improvements in the low temperature performance require consideration of the underlying components and principles of operation of nickel metal hydride batteries.
Nickel metal hydride batteries typically include a nickel hydroxide positive electrode, a negative electrode that incorporates a hydrogen storage alloy, a separator and an aqueous alkaline electrolyte. The positive and negative electrodes are housed in adjoining battery compartments that are typically separated by a non-woven, felled, nylon, polyethylene, or polypropylene separator. Several batteries may also be combined in series to form larger battery packs capable of providing higher powers, voltages or discharge rates.
The charging and discharging reactions of nickel metal hydride batteries have been discussed in the art and may be summarized as shown below:
Charging:                positive electrode: Ni(OH)2+OH−→NiOOH+H2O+e−        negative electrode: M+H2O+e−→MH+OH−        
Discharging:                positive electrode: NiOOH+H2O+e−→Ni(OH)2+OH−        negative electrode: MH+OH−→M+H2O+e−        
Much work has been completed over the past decade to improve the performance of nickel metal hydride batteries. Optimization of the batteries ultimately depends on controlling the rate, extent and efficiency of the charging and discharging reactions. Factors relevant to battery performance include the physical state, chemical composition, catalytic activity and other properties of the positive and negative electrode materials, the composition and concentration of the electrolyte, materials used as the separator, the operating conditions, and external environmental factors. Various factors related to the performance of the positive nickel hydroxide electrode have been considered, for example, in U.S. Pat. Nos. 5,348,822; 5,637,423; 5,905,003; 5,948,564; and 6,228,535 by the instant assignee, the disclosures of which are hereby incorporated by reference.
Work on suitable negative electrode materials has focused on intermetallic compounds as hydrogen storage alloys since the late 1950's when it was determined that the compound TiNi reversibly absorbed and desorbed hydrogen. Subsequent work has shown that intermetallic compounds having the general formulas AB, AB2 A2B and AB5, where A is a hydride forming element and B is a weak or non-hydride forming element, are able to reversibly absorb and desorb hydrogen. Consequently, most of the effort in developing negative electrodes has focused on hydrogen storage alloys having the AB, AB2, AB5 or A2B formula types.
Desirable properties of hydrogen storage alloys include: good hydrogen storage capabilities to achieve a high energy density and high battery capacity; thermodynamic properties suitable for the reversible absorption and desorption of hydrogen; low hydrogen equilibrium pressure; high electrochemical activity; fast discharge kinetics for high rate performance; high oxidation resistance; high resistance to cell self-discharge; and reproducible performance over many cycles. The chemical composition, physical state, electrode structure and battery configurations of hydrogen storage alloys as negative electrode materials in nickel metal hydride have been investigated and reported in the prior art. Some of this work is described in U.S. Pat. Nos. 4,716,088; 5,277,999; 5,536,591; 5,616,432; and 6,270,719 to the instant assignee, the disclosures of which are hereby incorporated by reference.
Efforts to date indicate that certain intermetallic compounds are capable of effectively functioning as negative electrode materials in rechargeable batteries, but that important properties are difficult to optimize simultaneously. Hydrogen storage alloys of the AB5 type, for example, generally have easier initial activation, good charge stability and relatively long charge-discharge cycle life, but at the same time have relatively low discharge capacity. Furthermore, attempts to increase the cycle life generally lead to reductions in the initial activation. Hydrogen storage alloys of the AB2 type, on the other hand, typically possess high discharge capacity, but low initial activation and relatively short cycle life. Efforts to improve upon the initial activation generally come at the expense of cycle life. Other important properties include discharge rate, discharge current, and constancy of energy or power delivery over time. It has proven difficult in most applications to simultaneously optimize all desired battery attributes and as a result, compromises are normally made in which some properties are sacrificed at the expense of others.
A need exists for improved rechargeable batteries having higher powers and discharge rates at low temperatures. With respect to nickel metal hydride batteries, the barrier to low temperature performance appears to reside primarily in the operating characteristics of the negative hydrogen storage alloy electrode. Consequently, a need exists for improving the performance of hydrogen storage alloys at low temperatures. New concepts in materials design are required to meet this need.